The invention relates to a system for providing the synchronization of a television receiver intended to receive a signal, more specifically a signal of the "MAC" type, for example, DMAC or D2MAC which conveys the luminance and analog color information components associated with periods of what is referred to as a duobinary signal, which signal comprises once in every frame analog frame reference voltage plateaus for the black, grey, white levels, and a duobinary portion containing more specifically, a synchronization signal, the "Frame synchronization word". The frame synchronization word passes through the system via a variable gain amplifier and via a continuous component aligning device, the system including a duobinary signal decoding device, a peak detector for measuring and storing the peak values of the signal and associated means for applying, on the basis of these values, to the aligning device a datum for the correction of the so-called continuous component and, to the amplifier, a datum for the gain control, a measuring arrangement for measuring the levels of said frame reference voltage plateaus of each frame and associated means for applying, based on these levels, a gain control value to the amplifier, and a word detector for recognizing the frame synchronization word.
The problem presented has its origin in the fact that, to ensure a fail-safe detection of the digital frame synchronization words, it is necessary to correctly check the level of the continuous component of the signal and its amplitude beforehand, while for the control of these values, one is to base oneself on the reference voltage plateau which cannot be used until after synchronization has been detected. These reference voltage plateaus will hereafter be called "plateaus".
Systems have been proposed, as for example in the European Patent Application EP-A 0.167.430, in which two consecutive operating modes are used. The peak values of the signal are first measured. These values are denoted Vmax and Vmin, values A1=Vmax-Vmin and B1=(Vmax+Vmin)/2 and can be calculated. The value A1 gives the range on the basis of which the gain of the variable-gain amplifier is regulated. The value B1 gives the continuous component and renders it possible to adjust the latter. Thanks to the coarse adjustment obtained during this first mode, the frame synchronization words can be detected and consequently, a synchronization can be obtained on the basis of which it is possible to find the plateaus to ensure accurate adjustments of the gain and the continuous component.
Using this system has several disadvantages; actually the recognition of the frame synchronization word is not a sure indication that synchronization has been reached, that is to say that there is perfect synchronization, and subsequent to the decision taken after the recognition of the frame synchronization word, several difficulties may become apparent, more specifically in the following cases:
it is assumed that synchronization has been obtained and that the system is really in synchronization: the change from one mode to the other is critical, sudden signal transients may occur.
it is assumed that synchronization has been obtained while actually there is no synchronization: in this case the plateaus do not appear at the instants at which they are expected and the adjustment based on them is faulty.
a confirmation of the synchronization is preferred and the recognition of a second frame synchronization word is waited for: it being possible that the content of the pictures may give rise to a faulty alignment, the recognition of a second word may be very difficult.